1. Field of the Invention
The invention relates to an improved microwave system for measuring the moisture content of cotton bales and other materials.
2. Description of the Prior Art
Modern cotton gins have the purpose of extracting lint (the cotton) from trash and seeds—usually the sticks, leaves and burrs that are entrained with the cotton. These modern gins include many individual machine components that are operated sequentially to form the gin processing line. The components are often specific in the types of trash that they remove. Stick machines, inclined cleaners, and especially lint cleaners process the lint to a purity where it can be baled and delivered to spinning mills. Included in these systems are drying systems as it is imperative that the cotton must be within a range of moisture contents in order for the machinery to work properly.
Unfortunately, the cotton processed by such machines varies widely in moisture content and to date the technology available to measure the moisture content is limited to inaccurate low cost resistance moisture sensors or extremely expensive microwave moisture sensors. Thus, only a very few cotton gins are even attempting to measure the moisture content even though control over the moisture content is critical to the operation of the cotton gin in terms of both productivity as well as for the preservation of the required quality standard for the ultimately produced lint cotton.
This need to produce a better quality product for sale to the cotton textile mills and to reduce labor costs during processing has led to considerable interest in process control for cotton gins. Anthony and Byler (1994) indicate that process control can range from $15,000 to $100,000. Most of the work to date has involved the online measurement of moisture and trash. It is inevitable that the cotton gins in the near future will become fully computerized and automated (Byler and Anthony, 1997). This is due to the fact that optimal control of the gin will produce optimal economic returns for a given ginned bale of cotton. This will be advantageous to the growers, the ginners, and the processing mills as they will receive a consistent product that can be tailored to their desired specifications. In this regard, it is expected that the gins will become fully automated in the near future as suitable low cost technology becomes available. It has already been shown that this automation will utilize some form of moisture measurement system at several key locations scattered throughout the ginning process.
Byler and Anthony (1997) reported on a computer-based cotton color and trash and moisture measurement system that was used to control the drying and cleaning machinery. This system utilizes a resistance sensor. The system can only be made to work when used in conjunction with a sampling system that presents a solid piece of lint (no voids or holes) and at a uniform packing density to remove the effect of varying lint density from the measurement. This system was installed at a gin in Cortland, Ala. in 1994. In 1994, it was reported to be the most complete computerized gin process control system in the world. This process control system utilized two trash and moisture level sensors. The cotton moisture/color/trash sensors were based upon the High-Volume-Instruments (HVI) that are used in the United States Department of Agriculture's Cotton Classing Office. The first sensor was located opposite of a ram located in the back of the feed control. The feed control is located before the gin stand where the lint is removed from the seed cotton. The ram was periodically extended to press cotton against a glass sample imaging and resistance sensing plate. The second color/trash/moisture measurement station was located after the gin stand and before the lint cleaners. A paddle sampler was used to obtain a sample from the duct and press the sample against a sensing window.
Byler (1992) reported that sample compression against a resistance sensing plate was used to increase the sample density in order to produce a more repeatable moisture reading by minimizing the sample density variations. The sample compression was felt to be important enough that several devices were developed to accomplish this and U.S. Pat. No. 5,125,279 Jun. 30, 1992 entitled System for Analyzing Cotton was obtained for a paddle sampler to accomplish the sample extraction compression for the trash, moisture and color measurement, as well as the moisture sensing patent U.S. Pat. No. 5,514,973 which is based upon resistance sensing. It is still in use to date in the Zellweger Uster Intelligin and was reported to be fully functional in two commercial gin's as conducted in a USDA study (Anthony et al., 1995).
Another disadvantage to this technique is the need for pressing the cotton against a resistance sensing plate, as this restricts the possible locations where this technique can be applied in a cotton gin in addition to the very likely possibility of stoppage/blockage of the cotton flow due to system malfunctions.
The final stage in the cotton processing stream is the cotton bale packaging system. Recent innovations has shown that the use of cotton moisture restoration systems both reduce stress on the pale packaging system as well as add additional weight to the bales. As cotton is sold on a wet basis there is a real market incentive for the utilization of these systems. As such, currently there are no resistive sensors that can be accurately used to control these systems as the surface moisture these systems add to the cotton alter the calibration of these sensors in an uncontrolled manner. The only other types of sensors that can be utilized are very expensive microwave bale moisture sensors that are typically utilizing very high frequency microwave technology that is prohibitively expensive to manufacture. Given this fact there are a large number of cotton gins that are using moisture restoration systems without any type of feed back control sensor to maintain the correct moisture in the cotton bales.
Cotton bales are provided an official grade that is based upon samples obtained immediately upon the baling of the cotton. These samples are shipped to a USDA-AMS classing office where both the color and trash content are measured. This grade is then used to set the value of that particular bale of cotton. Unfortunately, when moisture is added to the bale in an excessive amount, this grade has been shown to change as the moisture degrades the cotton. This has led to hundreds of bales being returned back to the gins and has caused a great deal of concern in the industry over the potential damage to the USDA-AMS classing grade. Given this situation it is critical that a low cost moisture measurement system is developed that will determine the moisture content of the cotton bales. This system could then be easily deployed at an affordable price to all gins that are using moisture restoration and will protect and maximize the quality and value of their cotton as well as preserve and protect the reputation of US cotton and USDA-AMS cotton grades.
In the field of non-contact moisture sensing, there exists two main styles of radio frequency sensing systems, near and far field detection. An example of a near field system which is based upon a very low frequency rf field measurement, U.S. Pat. No. 6,275,046. This system utilizes a near-field electric field measurement which to date has demonstrated poor repeatability, low accuracy and severe drift over time problems. Other more relevant instruments of note are of several microwave sensor patents.
U.S. Pat. No. 2,659,860 teaches a method to measure the moisture content of bales of material, by directing a 10 GHz microwave beam through the bale and receiving the beam with another antenna on the far side of the bale from the one which generated the signal. The moisture content of the bale is then determined solely from the attenuation of this signal.
Meyer and Schilz U.S. Pat. No. 4,361,801 teaches a sensing technique that requires measurements of both attenuation and the phase delay of propagation in order to calculate the real and the imaginary components of the complex permittivity measurement in order to measure moisture at 9 GHz which is independent of density. The basis for this measurement is the ratio of the complex permittivities providing, which is a modification of taking the ratio of the attenuation to the propagation delay, as the measure of moisture (either as phase delay or equivalently the time delay). Nelson et al. U.S. Pat. No. 6,147,503 describes another moisture sensor algorithm that provides a moisture sensor that is independent of density over the narrow range of densities provided by loose seed kernel samples versus tightly packed seed kernel samples. They teach a technique that operates at 11.3 and 18 GHz again using both the attenuation and the propagation delay to calculate the complex permittivity of the material to derive an algorithm for the determination of the moisture content of the material. Moshe et al. U.S. Pat. No. 6,476,619 describes a microwave cavity perturbation technique for the sensing of moisture and or density in cotton sliver that has a preferred operating range of 7–9 GHz. In the perturbation technique the system is setup with a resonant peak in the signal amplitude versus frequency plot and utilizes the frequency change in the location of this peak as the measure of permittivity change thereby providing a measure of the permittivity from which the moisture content can be estimated assuming a constant density of material. Moshe et al. U.S. Pat. No. 6,111,415 describes the use of the well known radar technique of Frequency Modulated Time Domain “FMTD” for use as a density sensor which is used to correct an attenuation based moisture sensor. Other patents by Moshe et al. include U.S. Pat. Nos. 5,845,529 and 6,107,809 which utilize a ratio of attenuation to phase delay measurement in a manner very similar to the Meyer and Schilz U.S. Pat. No. 4,361,801. The reoccurring theme between all of these patents is that they all use very high microwave frequencies, typically above 7 GHz, and all of them utilize a measure of the attenuation of the signal after it has been transmitted through the material under test as the primary measure of the moisture content. As such, all of these patents provide very expensive solutions. Additionally it should be noted that the radar cross-section of the typical metal bale ties is very large at these high microwave frequencies and has been shown to cause significant signal interference at these very high frequencies, thereby rendering all of these frequencies unusable for use in moisture measurement of metal tied cotton bales.
In today's modern cotton gins, all of the US cotton gins are housed in metal clad structures. When utilizing microwave and radio frequency based sensors in such buildings, standing waves are setup that cause significant interference with the measurement. The interference affects both the signal strength (attenuation) measurement as well as the propagation phase delay measurement. This interference degrades the accuracy of these systems and in most situations renders the technique unusable and makes it impossible to calibrate the system ahead of time at the factory. Thus, the units are shipped uncalibrated and must be installed with expensive and elaborate field calibration techniques that add significant cost to the system as the manufacturer must pass on this added cost of doing business. Thus, it is a primary goal of this invention to provide a method by which to produce a system that provides a measurement of moisture that is installation independent and is only dependant upon the material's permittivity. In order to accomplish this, the invention as described herein removes from the measurement the multi-path interferences and utilizes a low microwave frequency to avoid interference from spurious emissions from metal-tied cotton bales. It should be noted that as described this system will work equally well with non-metal bale tied cotton bales.